In recent years, remarkable technical development can be seen in those devices which diagnose/image the inner structure or function of an object by means of radiation, such as X-ray beams and gamma-ray beams. For this type of devices, detectors for detecting radiation are essential. Enhancement of performance of such radiation detectors also contributes to the above technical development. In particular, enhancement of performance is promoted in so-called digitization, with which a detection signal is outputted in digital form, as well as achievement of fine pixels and enlargement of a sensor surface.
As a detection method of such a radiation detector, a detection method called the photon counting method attracts attention, in addition to the conventionally used integration method (integration mode). The photon counting method has been applied to a gamma-ray detector in the field of nuclear medicine (e.g., see Patent Document 1: JP-A-H11-109040). On the other hand, according to some reported cases of recent years, the photon counting method is applied to an X-ray detector in order to achieve the effect of improving the image enhancement ability, reducing the metal artifacts, mitigating the impact of beam hardening, and the like.
As one of such application cases, the one suggested in Patent Document 2: JP-A-2006-101926 is well known. Specifically, the document suggests: “A radiation detection apparatus including a photon counting type detector that deals with radiation as photons, the radiation being incident on each of a plurality of acquisition pixels, to output an electrical signal according to the energy of the particles, wherein: based on the signal outputted by the detector for each acquisition pixel, the apparatus calculates a count data of the number of particles of the radiation, which are classified into a plurality of energy ranges in an energy spectrum of radiation; the apparatus performs weighting with respect to the count data of each of the plurality of energy ranges of each acquisition pixel subjected to the calculation, using a weight coefficient given to each of the energy ranges; the apparatus adds the count data of the respective plurality of energy ranges of each weighted acquisition pixel; and the apparatus outputs the addition data as radiation image generation data for each acquisition pixel”.
Thus, in a photon counting type X-ray detector, one or more thresholds (preferably, a plurality of thresholds) are prepared to discriminate the energy which is possessed by each incident X-ray photon. Since the range of energy is defined by the thresholds, the energy range to which the energy of each X-ray photon belongs can be determined. As a result of the determination, measurement is performed for the number of X-ray photons discriminated into the respective energy ranges. The information of the measured number is reflected to a pixel value of an image.
In the photon counting type X-ray detector, upon entry of X-ray photons into pixels (i.e. acquisition pixels), an electric pulse signal is outputted from each acquisition pixel. The energy of the X-ray photons incident on each pixel is reflected to a peak value of the pulse signal generated by the X-ray photons. The value of data outputted from each acquisition pixel depends on which of the thresholds the peak value exceeds. Therefore, each threshold is required to be retained with high accuracy with respect to the energy possessed by the X-ray photons and with as much evenness as possible between the acquisition pixels. The accuracy and the evenness are influenced by the sensitivity of each X-ray sensing element configuring an acquisition pixel, and by the difference in the characteristics of a circuit formed of a CMOS on an output channel side of each element. Therefore, it has been necessary to calibrate each acquisition pixel and each threshold of the pixel to adjust the sensitivity of each acquisition pixel with respect to the energy of X-ray photons so that the sensitivity will be the same or is considered to be the same between the acquisition pixels.
Conventionally, this calibration has been performed using a plurality of gamma-ray sealed radiation sources, such as 241-Am (59.5 keV) or 57-Co (122 keV) having a known energy value. Specifically, the radiation sources are placed in front of a sensor surface of an X-ray detector and gamma-ray beams are radiated for a predetermined period. The X-ray sensing element that has received radiation of the gamma-ray beams outputs an electric pulse in accordance with the known energy value. Using the signal value from each acquisition pixel, the thresholds given to each acquisition pixel are adjusted so that the sensitivity (generally called S-characteristics in which the output is distorted in lower intensities and higher intensities in the signal: amplitude of electric pulse relative to energy value is indicated) with respect to the energy value of X-ray photons will be substantially the same between the acquisition pixels, i.e. between acquisition channels.
An example of setting thresholds described in Non-patent Document 1 is also well known. In the example described in this document, one threshold is ensured to be given to a detector that uses CdTe.